Justin Bieber's very long week in London has finally come to an end, but not without making a few headlines. A source close to the singer has an explanation for at least some of Bieber's seemingly bizarre behavior, however.

"The gas mask Justin has been wearing around London is a joke between his friends, but people are starting to take it seriously," the source tells Us Weekly. "Justin's close friend Julian Keyz (a young songwriter/producer also known as Julkeys) also wears it to pretend he's Justin and throw people off as they're the same height."

"All his friends take turns wearing it as a joke to throw off the paparazzi," the source continued. "It's not a serious thing, they're just having fun around London with it."

The 19-year-old singer was first spotted wearing the odd mask several weeks ago while shopping in Londontown, and he wore the disguise a second time on Wednesday, March 6, as he left famed restaurant Mr. Chow's.

Just one week earlier, the "Boyfriend" singer turned heads for a different reason — wearing a bizarre ensemble of purple leopard-print harem pants, a black sweatshirt with illustrations of blue and yellow rolled-up bills, and a bright yellow cap with silver spikes.

Bieber later set the record straight with his fans, telling them that he had donned the unusual getup as part of a dare from his pal Ryan.

"Ryan said you won't go out in that, I said watch me," the prankster wrote on Instagram. "Haha, too colorful. Next time I'm gonna wear a black hat."

On Thursday, March 7, the singer was hospitalized after fainting mid-show during concert at London's O2 arena. After spending 20 minutes offstage recouping, he returned to the stage to finish off the show.